The invention concerns a heat exchanger which is to be installed in a nuclear reactor of the cavern type, the heat exchanger being located in a cavern with an impermeable steel liner, and having a large number of reaction tubes filled with reforming catalyst. The material to be subjected to reforming, mostly hydrocarbons, flows through this reforming catalyst. The reaction tubes are heated by the reactor recycle gas.
Heat exchangers of the type mentioned utilize the waste heat of a nuclear reactor in order to carry out chemical processes. With such heat exchangers, it is necessary to take into consideration the mode of operation of the catalytic high-temperature reaction and to conform to the safety specifications for installations with high-temperature nuclear reactors.
It is known that the nuclear fuel elements of a nuclear reactor are cooled with fluid, for example helium. The cooling fluid attains a temperature of 950.degree. C. This cooling fluid, hereafter referred to as recycle gas, is then withdrawn from the reactor where its temperature is lowered to about 200.degree. to 300.degree. by transferring its heat to one or more process fluids and then returned to the reactor core to be reheated.
The equipment in which the recycle gas circulates must be doubly protected against leakage toward the outside, since it inevitably contains a certain quantity of radioactive substances.
As is commonly known, the equipment that transfers the waste heat of the nuclear reactor, the heat exchanger, is arranged radially in caverns in the pre-stressed concrete that surrounds the nuclear reactor core. The caverns have a steel lining, or liner, that has at least one cover plate above and below. Appropriate reaction tubes are arranged in the heat exchanger if the waste heat of the nuclear reactor is to be used for carrying out chemical processes. The reaction tubes for the endothermic catalytic high temperature reaction, for example methane reforming, are filled with catalyst pellets. The gas to be reformed passes downwardly through the reaction tubes and is simultaneously heated with the hot recycle gas in order to maintain the endothermic reaction. Differing film and heat transfer coefficients occur inevitably because of irregularities in the catalyst filling and differing reaction rates and gas velocities.
The result is that the tube is subjected to a varying thermal stress and that, in places, this tube may be overheated and destroyed. Because of the specific high loading of the reaction tube and the quality of the material employed, the tubes cannot all have the same service life and some will become defective before others.
In view of the fact that a multitude of reaction tubes is located in a cavern of the pre-stressed concrete wall of a high temperature nuclear reactor, it is particularly important to be able to block the inlet and outlet pipes, either individually or in sections, so that damage to the reaction tubes will entail the shutting down only of the tube or section involved. Interim repairs are extremely difficult and lead to costly shutdown periods. In addition, it is necessary to provide a double seal toward the outside for the recycle gas stream. As is known, this is done by installing two cover closures one above the other in the steel lining. Inlet and outlet pipes must then be led through these covers or through side passages in the pre-stressed concrete vessel. For many reasons, this last solution is undesirable.
To make things even more difficult, the widely differing temperatures at the time of erection and in operation cause considerable thermal expansion to take place in the connecting pipes between erection and operation.